You asked for opinions, so here's mine. If the child is eating other foods well and does not have a history of a delicate digestive system, I would not hesitate to switch to whole milk. You definitely want to give him/her the extra fat in whole milk until 2 years of age. Also start vitamins, if you haven't already.
However, if the child is a light eater or a picky eater or has history of a delicate tummy, I would keep him/her on formula until at least 12 months. Just a bit of added insurance that he/she is getting everything they need.
Best wishes,
Peggy